Silicon carbide refractories



Nov. 18, 1958 H. F. G. UELTZ ETAL 2,860,999

SILICON CARBIDE REFRACTORIES Filed Dec. 16, 1954 2 Sheets-Sheet 1HERBERT FIG. L/ELTZ NEIL N. AuL T ATTORNEY l VENTORS.

Nov. 18, 1958 H. F. G. UELTZ ET AL 12,850,999

' SILICON CARBIDE REFRACTORIES Filed Dec. 16, 1954 2 Sheets-Sheet 2 70N0 0 RIM/4&0;

/ INVENTORS.

HERBERT F5. L/ELTZ NE/ 1. IV. A uz. T

United States PatentO SILICON CARBIDE REFRACTORIES Herbert F. G. Ueltz,Worcester, and Neil N. Ault, Holden, Mass., assignors to Norton Company,Worcester, Mass., a corporation of Massachusetts Application December16, 1954, Serial No. 475,846

8 Claims. (Cl. 106-44) The invention relates to silicon carbiderefractories.

One object of the invention is to provide a superior article of siliconcarbide for use as kiln furniture, for example for batts, plates,saggers and the like. Another object of the invention is to providesilicon carbide refractories which have superior characteristics for useas slag hole blocks. Another object of the invention is to providesilicon carbide refractories which have superior characteristics for useas tubes for furnaces. Another object of the invention is to providesilicon carbide refractories which have superior characteristics in allof the following respects: strength, lack of glaze, resistance tooxidation, resistance to thermal shock.

Other objects will be in part obvious or in part pointed outhereinafter.

Fig. 1 is a ternary composition diagram of a portion of the system KO--CaOSiO on which the compositions within the range of those defined bythe subsequent Table I are represented by the shaded area ABCD. Fig. 2is a ternary composition diagram of part of the system K OAl O --SiO onwhich compositions within the range of those defined by the data of thesubsequent Table III are represented by the shaded area EFGH.

Fig. 3 is a ternary composition diagram of a portion of the system NaO-CaO-SiO on which compositions within the range of those defined by thedata given on subsequent Table IV, are represented by the shaded areaFig; 4 is a ternary composition diagram of a portion of the system NaOAl O --SiO on which compositions within the range defined by dataonsubsequent Table V, are represented by the shaded area NOPQ.

We can use the ordinary black or grey variety of silicon carbidecrystallized in the hexagonal system. The purer -or green variety couldbe used but there is no particular advantage in using it. lnorder toobtain high density we prefer to use a mixture of grit sizes. .Apractical ex ample is 10 mesh and finer down to the irnpalpable fines.The objective is to obtain a maximum bulk density of the grain butwithin the scope of thisinvention any grit sizes the following points onthe ternary composition diagram of Figure 1:

TABLE I Composition Range in Parts by Weight cameo The liquidustemperatures of compositions within the limits-described above, rangefrom, about 750 C. to 1250" C. Our definition of liquidus temperature isthe temperature under conditions of chemical equilibrium above which thesystem is completely liquid and at which and below which at least onecrystalline phase is present in the system.

After mixing the silicon carbide grain with the bonding glass and with atemporary binder such as a solution of dextrine and water, we charge themixture into a mold and form the desired article by pressure in theusual way. Or on the other hand we can ram the mixture. Any moldingtechnique can be used. After the thus formed shape has been removed fromthe mold and allowed to dry it is fired in a kiln to cone 16 and thiscompletes the manufacture of the product. Usually the top temperature ofa cone 16 burn, which varies as is well known in the art dependent uponthe length of the soaking period, is around 1450 C. It will readily beseen, therefore, that with a glass bond having a liquidus temperature of1000" C., the bond would, if there were no chemical reaction, be highlyfluid during firing. We have found that by using a very small amount ofglass bond, which has'a low liquidus temperature relative to the soakingtemperature during firing, the glass dissolves silica formed byoxidation of some of the silicon carbide, and as the silica content ofthe bond increases, its liquidus temperature increases. The originalliquidus temperature of our bond is in the range of temperature at whichsilicon carbide begins to oxidize. When this temperature is reached thevery fluid glass, wets the silicon carbide grains early in the firingcycle. One point is that the smaller silicon carbide grains meaningespecially the very fine material starts to oxidize first and normallysuch oxidation product is a fine fluify material, around thistemperature range of the liquidus temperature given and just above, andthis fiuify material would prevent the formation of a silicon carbiderefractory having the best properties. But, as in accordance with ourinvention, this fluffy silica dissolves in the glass as it is formed,there is formed only a negligible amount of such low density crystallinesilica which would otherwise tend to separate the grains of siliconcarbide and produce a low density product. I

We find further advantages in using a very small amount of glass with alow liquidus temperature for the manufacture of a silicon carbiderefractory fired to cone 16 or thereabouts. The glass formed does notmigrate to the surface and form a glaze. The bond is refractory andgives good strength at high temperatures. The high density productsobtained have high resistance to oxidation.

As proof of our theories of what happens in the firing of our products,we have checked the index of refraction of the glass as a glass and theindex of refraction of the glassy bond in our fired product and comparethat with the index of refraction of pure SiO glass which is 1.459. Theindex of refraction of the glass we use in its preferred composition,which has the molecular formula K O-2CaO-9SiO is 1.518 and the index ofrefraction of the glassy bond in the fired product using this preferredglass is about 1.47 6. From this it is evident that there has been aconsiderable solution of SiO formed by oxidation of SiC and it isestimated that the fired bond has a liquidus temperature of at least1600 C. This temperature is considerably in excess of the firingtemperature at soak and in excess of the temperatures at which theproduct will be used.

The important characteristics of our bond are: (1) that our bondmaterial which we add to the mixture forms a liquid early in thefiringcycle as the silicon carbide grain begins to oxidize to form silica, and(2) our bond material plus the silica formed by oxidation has asignificantly Patented Nov. 18, 1958' higher liquidus temperature thanthe original glass bond. Our glass derived from the bond material shouldhave a liquidus temperature in the range from 700 C. to .1250 C. and beof such a composition that the liquidus 4- the composition of theoriginal glass plus the silica formed by oxidation is about 1600 C.

Example 2 temperature of the original glass plus the silica formed byoxidation of silicon carbide is at least 1450 C. For a p mp of theSecond embodlment of the bond compositions described in Table I, onlythree 0111' Invent-i011 Involving the glass Table III, We madecomponents are shown other constituents can l b silicon carbide battsout ofblack SlllCOIl carbide of the present if the characteristicsdescribed above are main- Same range 015 grit 51168 as 111 Table tained.Using 0.5 part by weight of the preferred glass com- While we prefer tofire our refractories to cone 16, Position, 2 10% 2 3 and.80% a Wlth 995good results according to the invention can be achieved Parts y Wfflght0f the slllcon Carblde fnlxture and if they are fired to anywhere fromcone 14 to cone 20. Parts y Welght Walter and 2 Paris y Welght 0fPowdered As known i is far more accurate and i tifi to dextrinedissolved therein, we pressed batts of several d fi the firing bpywmetric cones than by tempera. 15 different sizes under a pressure oftwo and a half tons tures as results can be better duplicated bymeasuring P Square inch- After drylng these were f 111 a thevitrification by theme f standard cone5 cone 16 kiln. These batts hadall the superior properties listed in the objects and were found to begen- Example 1 erally superior to silicon carbide batts previously manu-20 factured.

As a specific example of our invention, we made silicon n an t eembOdilIlent Of Our invention, We Provide carbide batts out of graysilicon carbide of the following bonding glass in the form of a fritwhich is crushefi to range f it i a fine powder preferably and which hasa composition TABLE 11 within the compositional area described by thefollow- Weight percent ing points on the ternary composition diagram ofFig- ;10 to 24 mesh r l 33 re to 90 mesh 33 100 mesh and finer e 34TABLE IV 100 30 Composition Range,

Parts by Weight Using 0.5 part by weight of the preferred glass com- 7position K O-2CaO-9SiO with 99.5 parts by weight of 15810 the slliconcarbide mixture plus 4.5 parts by Weight of s10, 70 6G 71 83 water and 2parts by weight of powdered dextrine dis- 35 7 solved therein, wepressed batts of several different sizes under ressure two a d half toner uar in h.

After ri ing thesg were fi ied in a con: I 16 lii ln. Th se The i gtemperatui'es compositions wlthm flu batts had all the superiorproperties listed in the objects comoposmon described In Table Iv vary rand were found to be generally superior to silicon carbide 40 We i t lowpercentage of batts previously manufactured this soda-lime glass, ourlimits being from 0.1% to In another embodiment of our invention, weprovide 20% glass b by.welght of the total refriictory' bonding glass inthe .form of a frit which is crushed to our preferred percentage is 05%of glas? weight. of a fine powder preferably and which has a compositionthe Ital i i piocedure of mlxmg molding with the compositional areadescribed by the following l g g ffi s 1 5 2 5 1 5 22 ;225:25 ii j s gigg points on the ternary composltlon dlagram of Flgure has the samecharacteristics as previously described for the glass of Table I. Thepreferred glass of this em- TABLE III bodiment is 11.4% Na O, 15.6% CaO,and 73% SiO percentages being by weight. The liquidus temperature ofthis composition is 1110 C. The previous remarks about otherconstituents and the previous remarks about firing to from cone 14 tocone 20, preferably to cone 16, 24 1O 16 apply. The liquidus temperatureof the composition of 8 ;g 2 2 55 the original glass plus the silicaformed by oxidation is at least 1600 C.

Example3 The liquidus temperatures of compositions within As a specificexample of the third embodiment of our the composition range describedin Table HI vary from invention involving the glass of Table IV, we madesilicon 850 C. to 1250 C. We use a very low percentage of carbide battsout of black silicon carbide of the same this potash-alumina glass, ourlimits being from 0.1% range of grit sizes asin Table H. to 2.0% of theglass bond by weight of the total re- Using 0.5 part byweight of thepreferred glass comfractor'y. Our preferred percentage is 0.5% of glassposition, 11.4% Na- O, 15.6% CaO and 73.0% SiO, with by weight of thetotal refractory. The procedure of 5 99.5 parts by weight of the siliconcarbide mixture and mixing, molding, pressing and/ or ramming is thesame 4.5 parts by Weight water and 2 parts by weight powdered as alreadydescribed. This glass in its reaction with dextrine dissolved therein,we pressed batts of several silicon carbide has the same characteristicsas previously different sizes under a pressure of two and a half tonsdescribed for the glass of Table I. The preferred glass per square inch.After drying these were fired in a cone of this embodiment is 10% K 0,10% A1 0 and 80% 16 kiln. These batts had all the superior propertiesSio percentages being by weight. The liquidus temlisted in the objectsand were found to be generally perature of this composition is about1050 C. The presuperior to silicon carbide batts previouslymanufactured.

-'vious remarks about other constituents and the previous In anotherembodiment of our invention, we provide remarks about firing to fromcone 14 to cone 20, prefbonding glass in the form of a frit which iscrushed to 'ei'ably to cone 16, apply. The liquidus temperature of atime powder preferably and which has a composition within thecompositional area described by the following points on the ternarycomposition diagram of Figure 4:

TABLE V Composition Range, Parts by Weight N850 24 10 18 A1201 0 14 12 0Bio. 76 76 83 82 of Table I. The preferred glass of this embodiment isNa O, 10% A1 0 and 80% SiO percentages being by weight. The liquidustemperature of this composition is about 1120 C. The previous remarksabout other constituents and the previous remarks about firing to fromcone 14 to cone 20, preferably to cone 16, apply. The liquidustemperature of the composition of the originacl: glass plus the silicaformed by oxidation is about 1550 Example 4 As a specific example of thefourth embodiment of our invention involving the glass of Table V, wemade silicon carbide batts out of black silicon carbide of the samerange of grit sizes as in Table II.

Using 0.5 part by weight of the preferred composition, 10% Na O, 10% A10 and 80% SiO with 99.5 parts by weight of the silicon carbide mixtureand 4.5 parts by weight water and 2 parts by weight of powdered dextrinedissolved therein, we pressed two batts of several different sizes undera pressure of two and a half tons per square inch. After drying thesewere fired in a cone 16 kiln. These batts had all the superiorproperties listed in the objects and were found to be generally superiorto silicon carbide batts previously manufactured.

Other formulae can be used for the silicate glass provided the reactiondescribed occurs during firing and the silicate glass originally has aliquidus temperature of from 700 C. to 1250 C. and the bond formed bythe reaction between the silicate glass and the silica formed byoxidation of a part of the silicon carbide has a liquidus temperaturebetween 1450 C. and 1700 C. However, so far as the process is concerned,our invention involves reacting a mixture including silicate glass withsilicon carbide to raise the liquidus temperature of the oxideconstituents to from less than 1250 C. to at least 1450 C. in the firedarticle.

In certain of the claims the composition is defined in ranges of severalcomponents in which each component is given four limits. This definesthe composition on the basis of a ternary diagram which is the mostaccurate scientific way of defining ternary compositions. It describesan area in a ternary diagram to a ceramist which fixes the limits of thecomposition and in each case it includes all the compositions of thecomponents mentioned which have the characteristics mentioned.

It will thus be seen that there has been provided by this inventionsilicon carbide refractories in which the various objects hereinaboveset forth together with many thoroughlypractical advantages aresuccessfully achieved.

As many possible embodiments may be made in the.

above invention and as many changes might be made in the embodimentsabove set forth, it is to be understood liquidus temperature of from 750that all matter hereinbefore set forth is to be interpretedasillustrative and notinalimiting sense.

We claim:

1. Silicon carbide refractory article which is the reaction product of amixture of from 98 to 99.9 parts by weight of silicon carbide with from2.0 to 0.1 parts by weight. of silicate glass which before firing has aC. to 1250 C. and which has a composition within the compositionrangedesignated by the shaded area ABCD in Figure l and Said silicateglassafter firing having a liquidus temperatureof from 1450" C. to 1700C., all parts being by weight. 7

2. Silicon carbide refractory article which is the reaction product of amixture of from 98 to 99.9 parts of silicon carbide with from 2.0 to 0.1parts of silicate glass which before firing has a liquidus temperatureof from 850 C. to 1250" C. and which has a composition within the limitsof composition range designated by the shaded area EFGH in Figure 2 andsaid silicate glass after firing having a liquidus temperature of from1450 C. to 1700' 0., all parts being by weight.

3. Silicon carbide refractory article which is the re action product ofa mixture of from 2.0 parts to 0.1 part of silicate glass which beforefiring has a liquidus temperature of from 790 C. to 1250 C. and whichhas a composition within the limits of composition range designated bythe shaded area JKLM in Figure 3, and said silicate glass after firinghaving a liquidus temperature of from 1450 C. to 1700 C., all partsbeing by weight.

4. Silicon carbide refractory article which is the reaction product of amixture of from 98 parts to 99.9 parts of silicon carbide with from 2.0parts to 0.1 part of silicate glass which before firing has a liquidustemperature of from 800 C. to 1250 C. and which has a composition withinthe limits of composition range designated by the shaded area NOPQ inFigure 4, and said silicate glass after firing having a liquidustemperature of from 1450 C. to 1700 C., all parts being by weight.

5. The process of making a refractory silicon carbide article whichcomprises molding, drying and firing under oxidizing firing conditionsto from pyrometric cone 14 to pyrometric cone 20 a mixture of from 98 to99.9 parts by weight of silicon carbide with from 2.0 to 0.1 parts byweight of silicate glass having before firing a liquidus temperature offrom 700 C. to 1250 0., thereby causing a reaction between the silicateglass and the silica formed by oxidation of a part of the siliconcarbide which adds to the glass and raises the liquidus temperature ofthe oxide constituents to from less than 1250 C. to at least 1450 C. inthe fired article, said silicate glass having before firing acomposition within the range designated by the shaded area ABCD inFigure l.

6. The process of making a refractory silicon carbide article whichcomprises molding, drying and firing under oxidizing firing conditionsto from pyrometric cone 14 to pyrometric cone 20 a mixture of from 98 to99.9 parts by weight of silicon carbide with from 2.0 to 0.1 parts byweight of silicate glass having before firing a liquidus temperature offrom 700 C. to 1250 C., thereby causing a reaction between the silicateglass and the silica formed by oxidation of a part of the siliconcarbide which adds to the glass and raises the liquidus temperature ofthe oxide constituents to from less than 1250 C. to at least 1450 C. inthe fired article, said silicate glass having before firing acomposition within the range designated by the shaded area EFGH inFigure 2.

7. The process of making a refractory silicon carbide article whichcomprises molding, drying and firing under oxidizing firing conditionsto from pyrometric cone 14 to pyrometric cone 20 -a mixture of from 98to 99.9 parts by weight of silicon carbide with from 2.0 to 0.1 parts byweight of silicate glass having before firing a liquidus temperature offrom 700 C. to 1250 C., thereby causing a reaction between the silicateglass and the silica formed byoxidation of a part of' the siliconcarbide which adds to the glass and raises the liquidus temperature ofthe oxide constituents to from less than 1250 C. to at least 1450 C. inthe fired article, said silicate glass having before firing acomposition within the range designated by the shaded area JKLM inFigure 3. i

8. The process of making a refractory silicon carbide article whichcomprises molding, drying and firing under oxidizing firing conditionsto from pyrornetric cone 14 to pyrometric cone 20 a mixture of from 98to 99.9 parts by weight of silicon carbide with from 2.0 to 0.1parts bywei'ght of silicate glass having before firing a liquidus temperature offrom 700 C. to 1250" (3., thereby causinga reaction between the silicateglass and the silica formed by oxidation of a part of the siliconcarbide which adds to the glass and raises the liquidus temperature ofthe oxide constituents to from less than 1250" C. to at least 1450* C.in the fired article, said silicate glass having before firing acomposition within the range designated by the shaded area NOPQ inFigure 4.

References Cited in the file of this patent UNITED STATES PATENTS1,105,070 Gage July 28, 1914 1,546,616 Buberl "July 21, 1925 1,546,833Geiger July 21, 1925 2,079,110 Easter et a1. May 4, 1937 FOREIGN PATENTS508,949 France Aug. 5, 1920

1. SILICON CARBIDE REFRACTORY ARTICLE WHICH IS THE REACTION PRODUCT OF AMIXTURE OF FROM 98 TO 99.9 PARTS BY WEIGHT OF SILICON CARBIDE WITH FROM2.0 TO 0.1 PARTS BY WEIGHT OF SILICATE GLASS WHICH BEFORE FIRING HAS ALIQUIDUS TEMPERATURE OF FROM 750*C. TO 1250*C. AND WHICH HAS ACOMPOSITION WITHIN THE COMPOSITION RANGE DESIGNATED BY THE SHADED AREAABCD IN FIGURE 1 AND SAID SILICATE GLASS AFTER FIRING HAVING A LIQUIDUSTEMPERATURE OF FROM 1450*C. TO 1700*C., ALL PARTS BEING BY WEIGHT.